Flat panel displays characterized by small thickness, low weight, and low power consumption find increasing use as displays in portable devices. Among flat panel displays of various types, liquid crystal display (LCD) devices are most widely used in laptop computers and desktop monitors because of their superior resolution, color image, and display quality.
LCD devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce a predetermined image. Liquid crystal molecules have a definite orientation, which depends on their particular characteristics. This orientation can be modified by an electric field applied across the axes of liquid crystal molecules. In other words, electric fields applied across the molecular axis can change the orientation of liquid crystal molecules. Due to optical anisotropy, incident light is refracted according to the orientation of liquid crystal molecules.
LCD devices comprise upper and lower substrates with electrodes, which are spaced apart and face each other, and a layer of liquid-crystalline substance confined between these substrates. When a voltage is applied to the liquid crystal layer with the aid of electrodes placed on each substrate, the direction of alignment of the liquid crystal molecules is changed in accordance with the applied voltage so as to display desired images. By controlling the applied voltage, it is possible to provide variable transmission of light beams in the LCD device to display data images.
LCD devices, however, do not emit light by themselves, and only control the transmission of incident light. Therefore, each LCD device needs an additional light source. In particular, LCD devices often use light sources in the form of backlight modules. The backlight modules are classified into “direct backlight” (or simply “direct”) and “edge backlight” (or simply “edge”) types, according to the arrangement of a lamp (or lamps). When a backlight module of the direct type is used for a liquid crystal display device, light beams radiated by a lamp are directly incident upon the LCD panel. In the case when the edge type backlight module is used, light beams radiated by a lamp enter the LCD panel via a light guide or a reflector. Light guides are made of optically transparent materials transmitting light along their length by means of total internal reflection. Finally, the light beam is reflected from the back surface of the light guide toward the front surface at an angle, which allows it to exit from the front edge of the light guide. Various reflection mechanisms are used to distribute the light exiting the guide uniformly over the panel, including reflective dots, channels, facets, etc.
Backlight modules employing non-collimated light sources, such as fluorescent lamps, typically incorporate at least two reflectors. A lamp cavity reflector is used to reflect the light emitted in a direction away from the light guide back toward the guide. This reflector can be specular or diffuse, although specular reflectors are most frequently employed.
The second reflector is provided near the back surface of the light guide to reflect light from the back surface of the light guide and direct it toward the front surface of the light guide where it can be transmitted to the viewer (to the LCD device).
The main disadvantage of conventional reflectors used in the lamp cavity and at the back surface of the light guide is the relatively high optical absorption and high transmission of incident light. Typical reflectors absorb or transmit from about 4 to 15% of the incident light. This fraction of light is obviously not available to the viewer; thereby the absorption and/or transmission lead to degradation of the backlight module performance.
With advent of the information technology age, there is increasing demand for high-quality LCDs. High-quality imaging requires more effective utilization of the radiation emitted by light sources. Conventionally, for obtaining singly polarized light beam output from an LCD backlight module, a polarizing plate is arranged between the LCD device and the backlight module in order to inhibit the passage of light beams with undesired polarization. In practice, the intensity of the resultant light beam with a single polarization state is less than half of the original light beam in terms of illuminance.